1. Field of the Invention
The field of the invention is that of systems for the transmission of signals between moving stations. More specifically, the invention is aimed at providing an optical linking system that can be used by air or space vehicles, for example to meet the requirements of communications between aircraft or between aircraft and the ground.
2. Description of the Prior Art
The presently existing air communications systems commonly use RF transmission, and have a number of drawbacks. Electromagnetic waves are sensitive to jamming caused by third-party transmissions as well as by transmissions from onboard equipment. Other limitations relate to heavy traffic in the transmission bands available and the small bandwidth that may be allocated to each communication. Finally, electromagnetic waves have the additional drawback of lacking discretion and, in certain situations, of giving vulnerability to aircraft that seek to be undetectable.
It is known that optical links may constitute an advantageous substitute for RF links, notably from the viewpoint of the above limitations. Indeed, the advantage of optical communications is that they have sufficient, but not unnecessarily excessive, range for air applications, have little susceptibility to external aggression and disturbance, including attempts at intrusion, and are discreet. Furthermore, the optical link generally provides for the availability of a high usable passband.
However, the establishment of optical communications between moving stations, notably between air vehicles, entails the combining of means that can be used to cope with the following constraints:
the system should provide directional search means that can be used to carry out an aiming of the transmitter during a phase when the link is initialized,
the system should provide quality aiming in order to maintain the link; the servo-control of the linking should notably enable the link to be maintained in spite of the substantial degree of relative motion that may exist among aircraft;
the range of the link should be sufficient to meet operational needs;
after an interruption of a link by a transient masking, it should be possible to reinitialize the link speedily;
the system should comply with the general specifications of equipment in aircraft;
the space occupied by the system should be compatible with its onboard use.